1. Field of the Invention
The present invention relates to light-emitting devices and methods of manufacturing the light-emitting devices. The present invention also relates to evaporation donor substrates which are used for deposition of a material which can be deposited onto a substrate.
2. Description of the Related Art
Organic compounds can take various structures compared with inorganic compounds, and it is possible to synthesize materials having various functions by appropriate molecular design. Because of these advantages, photo electronics and electronics which utilize functional organic materials have been attracting attention in recent years.
Solar cells, light-emitting elements, organic transistors, and the like are given as examples of electronics devices using an organic compound as a functional organic material. These devices take advantage of electrical properties and optical properties of the organic compound. Among them, in particular, light-emitting elements have been making remarkable progress.
A light emission mechanism of a light emitting element is described below: when voltage is applied between a pair of electrodes with an electroluminescence (hereinafter, also referred to as EL) layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined at an emission center in the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state and thus light is emitted. As excited states, a singlet excited state and a triplet excited state are known, and light emission is possible through either of these excited states.
An EL layer included in a light emitting element has at least a light emitting layer. The EL layer can have a stacked-layer structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and/or the like in addition to the light emitting layer.
EL materials for forming an EL layer are broadly classified into a low molecular (a monomer) material and a high molecular (a polymer) material. In general, a film of a low molecular material is often formed with an evaporation apparatus and a film of a high molecular material is often formed by an inkjet method or the like. A conventional evaporation apparatus has a substrate holder on which a substrate is placed, a crucible (or an evaporation boat) containing an EL material, that is, an evaporation material, a heater for heating the EL material in the crucible, and a shutter for preventing the EL material from being scattered during sublimation. The EL material which is heated by the heater is sublimated and deposited onto the substrate. At this time, in order to achieve uniform deposition, a deposition target substrate needs to be rotated, and the distance between the substrate and the crucible needs to be about 1 m even for a 300 mm by 360 mm substrate.
In the case of manufacturing a full-color flat panel display using emission colors of red, green, and blue by the above method, a metal mask is provided between the substrate and an evaporation source so as to be in contact with the substrate. Selective coloring can be achieved with this mask. However, this method does not provide very highly accurate deposition and thus requires that the distance between pixels be designed to be large and that the width of a partition wall (a bank) formed of an insulator between pixels be large. Therefore, application of the method to a high-definition display device is difficult.
In addition, demands for higher definition, higher aperture ratio, and higher reliability of a full-color flat panel display using emission colors of red, green, and blue have been increasing. Such demands are major issues in advancing miniaturization of display pixel pitches, which is associated with improvement in definition (an increase in the number of pixels) and a reduction in size of a light-emitting device. At the same time, demands for more productivity and lower cost have also been increasing.
Thus, a method of forming an EL layer of a light-emitting element by laser thermal transfer has been proposed (see Reference 1: Japanese Published Patent Application No. 2006-309994). Reference 1 describes a method for forming an EL layer in which a transfer substrate including a transfer layer formed over a supporting substrate with a photothermal conversion layer therebetween and an element-forming substrate are disposed to face each other, and the transfer layer is transferred to the element-forming substrate by irradiation of the transfer substrate with laser light (hereinafter, also referred to as a laser beam).
In addition, reference 1 discloses a method for improving the use efficiency of the transfer layer provided over the transfer substrate in such a manner that the transfer substrate after transfer of the transfer layer to the element-forming substrate is used again for transfer to another element-forming substrate.